1. Field of the Invention
This invention relates to the field of circuit protective devices, and in particular, to a circuit protection device having a relatively thin construction.
2. Technical Background
The demand for electrical power is insatiable because of the increased reliance on electricity for everyday needs. Power is provided to electricity users by way of electrical distribution systems that typically include electrical wiring from a utility power source to a breaker panel disposed in a house, building or some other facility. The breaker panel distributes AC power to one or more branch electric circuits installed in the structure. The electric circuits may typically include one or more receptacle outlets and may further transmit AC power to one or more electrically powered devices, commonly referred to in the art as load circuits. The receptacle outlets provide power to user-accessible loads that include a power cord and plug, the plug being insertable into the receptacle outlet. However, certain types of faults have been known to occur in electrical wiring systems. Accordingly, each electric circuit typically employs one or more electric circuit protection devices.
Both receptacle devices and electric circuit protective devices are disposed in an electrically non-conductive housing. The housing also includes electrical terminals that are electrically insulated from each other. The terminals provide a means for connecting the device to the source of AC power and a means for connecting the device to a load. In particular, line terminals couple the device to the source of AC electrical power, whereas load terminals couple power to the load. Of course, those of ordinary skill in the art will understand that the term “load” may refer to an appliance, a switch, or some other device. Load terminals may also be referred to as “feed-through” or “downstream” terminals because the wires connected to these terminals may be coupled to a daisy-chained configuration of receptacles or switches. The load may ultimately be connected at the far end of this arrangement. Referring back to the device housing, the load terminals may be connected to an electrically conductive path that is also connected to a set of receptacle contacts. The receptacle contacts are in communication with receptacle openings disposed on the face of the housing. This arrangement allows a user to insert an appliance plug into the receptacle opening to thereby energize the device.
With regard to installation, protective devices are commonly installed in outlets boxes. An outlet box may be located in a wall, ceiling, floor, counter-top, or the like. An electrical cable is placed from the breaker panel to the outlet box to provide the line terminals with AC power. The cable typically includes a plurality of insulated electrical conductors. Cables may be bundled together using a rigid or flexible tube made out of metal and/or electrically non-conductive material. A second cable of similar composition to the first cable is placed between the outlet box and any subsequent devices in the daisy-chain arrangement referred to above. The second cable, of course, is connected to the feed-through terminals. During installation of the outlet box, the cables are fed through openings in the outlet box for connection to their respective terminals, i.e., line or load. After the electrical conductors have been connected, the protective device is inserted into the front opening of the outlet box until the strap and a mounting surface in the outlet box mate.
One of the problems associated with device installation relates to the limited interior volume in an outlet box. When the protective device is inserted into the outlet box, the wires, cables and associated tubing disposed inside the outlet box must necessarily be compressed within the space formed between the back side of the device and the interior wall of the outlet box. This may lead to a number of adverse and undesirable results.
When an installer jams the device and the wires into the outlet box, the insertion force may cause the strap or some other member of the outlet box or protective device to deform. The deformation may interfere with the installation of the plate, or may prevent the protective device from fully coupling to the mounting surface of the outlet box. The deformation may also damage the protective device. As a result, the protective device may not function. Furthermore, the wire insulation may be urged against sharp interior edges of the outlet box or exterior edges of the protective device causing the conductors to become exposed. If the insulation loses its integrity, the electrical conductors may short together, or may short to the outlet box. Compression may also cause the insulation to split if the conductor becomes compressed within a tight bending radius. On the other hand, while any given electrical terminal is configured to grip electrical conductors with a securing force, the compression may apply an opposing force that results in loss of the intended electrical connection.
The above described adverse effects are dependent on the random motions of the electrical conductors while the protective device is being inserted in the outlet box. The chance occurrence of an adverse effect is aggravated by the fact that the compressed wires cannot be seen by the installer because they are hidden from view as the device is being inserted into the outlet box.
The problems described above are being exacerbated by the changes to wiring and installation practices that have occurred in recent years. The number of outlet box locations that require protection has expanded in the various commercial, institutional and residential sectors. Considering the residential sector, GFCIs were originally required to protect receptacles in the vicinity of outdoor swimming pools and have progressively been required to protect bathrooms, kitchens, basements and outdoor receptacles. More recently, AFCIs have been required to protect bedroom receptacles. The proliferation of installed locations increases the likelihood of such problems occurring.
At the same time, there has been a reduction in the thickness of the wall stud and sheetrock used to construct walls. This has necessitated the use of shallower outlet boxes. Unfortunately, the shallower outlet box provides less volume for the electrical conductors inside the outlet box, causing an increase in the compression forces on the electrical conductors. This development is further exacerbated by the increased use of multiple cables in the outlet box. The additional cables are needed for the redundant line and/or feed-thru terminals often included in the protective device.
In the residential market, the average square footage of new residences has been ever increasing. New residences typically include more built-in appliances than do older homes. As such, an increased amount of electric current must be propagated over larger distances. Accordingly, electrical conductors of greater cross section are required to conduct the greater current over a greater distance. Obviously, the consequence of multiple cables or electrical conductors of greater cross section is an increased probability of a problem occurring during installation.
Another problem occurs when new protective devices are used to replace older non-protective wiring devices in older homes. Note that the electrical conductors may be original to the house. The insulation associated with the original conductors may be weakened through aging. This may result in an older installation being more susceptible to one or more adverse effects described above.
All of the aforementioned trends lead to smaller outlet boxes. Accordingly, a decrease in the size of the protective device would be quite desirable. However, this is problematic because the necessary functionality that modern devices must possess is driver toward larger devices. To illustrate this point, a short survey of modern protective devices is provided below.
As noted above, there are several types of electric circuit protection devices. For example, such devices include ground fault circuit interrupters (GFCIs), ground-fault equipment protectors (GFEPs), and arc fault circuit interrupters (AFCIs). This list includes representative examples and is not meant to be exhaustive. Some devices include both GFCIs and AFCIs. As their names suggest, arc fault circuit interrupters (AFCIs), ground-fault equipment protectors (GFEPs) and ground fault circuit interrupters (GFCIs) perform different functions.
An arc fault is a discharge of electricity between two or more conductors. An arc fault may be caused by damaged insulation on the hot line conductor or neutral line conductor, or on both the hot line conductor and the neutral line conductor. The damaged insulation may cause a low power arc between the two conductors and a fire may result. An arc fault typically manifests itself as a high frequency current signal. Accordingly, an AFCI may be configured to detect various high frequency signals and de-energize the electrical circuit in response thereto.
Ground fault circuit equipment protectors (GFEPs) and ground fault circuit interrupters (GFCIs), on the other hand, are used to detect ground faults. A ground fault occurs when a current carrying (hot) conductor creates an unintended current path to ground. A differential current is created between the hot/neutral conductors because some of the current flowing in the circuit is diverted into the unintended current path. The unintended current path represents an electrical shock hazard. Ground faults, as well as arc faults, may also result in fire. GFCIs intended to prevent fire have been called ground-fault equipment protectors (GFEPs).
In addition to detecting arc faults and/or ground faults, a protective device itself must be protected from transient voltages and other surge phenomena. Transient voltages may be generated in a number of ways. For example, transient voltages may be generated by lightning storms. Transient voltages may also be produced when an inductive load coupled to the electrical distribution system is turned off, or by a motor coupled to the electrical distribution system that includes commutators and brushes. Whatever the cause, transient voltages are known to damage a protective device/cause an end of life condition. The damage may result in the protective device permanently denying power to the protected portion of the electric circuit. Consequently, the user suffers an expense and inconvenience of having to replace the protection device. Alternatively, the damage may result in the protection device becoming non-protective while continuing to provide power to the load circuit. The user can decide to keep using the device even though protection is not being afforded. Thus, damage of either type is not desirable. Accordingly, transient voltage tests are included in Underwriters Laboratories requirements for protective devices (e.g., UL standard 943 for GFCIs and UL standard 1699 for AFCIs). The protection device must continue to operate following these tests.
To meet the UL requirements, metal oxide varistors (MOVs) are typically employed. MOVs clamp the transient voltage imposed on the line (or load) terminals of the protection device to a safe voltage, i.e., a magnitude of typically not more than twice the phase voltage. One drawback to using MOVs relates to the fact that they are bulky and expensive. UL has recently promulgated new surge voltage requirements for GFCIs and AFCIs that test the protective device's ability to provide protection following exposure to harsher surge energy levels. The new requirement is typically met by including a larger MOV which is adept at absorbing the higher energy voltage impulses. The protective device may include other surge protection components. In addition to MOVs, other surge protective components such as transient voltage surge suppressors (TVSS), spark gaps, and other such devices may be used. These components are not meant to be an exhaustive list.
Of course, the circuitry used to implement a GFCI or an AFCI may typically include a sensor, such as a transformer, a solenoid, an SCR device, and other components disposed on a printed circuit board. The AFCI is a more recent protective device technology, similar to GFCI technology but typically having a detector that includes a greater number of electronic components that occupy more space. Those skilled in the art will recognize that there are different types of arc fault conditions, exemplified by a number of UL test conditions. There is a desire for a single protective device that passes as many of the arc fault test requirements as possible. There is also an increasing interest in combining two or more of a GFCI, TVSS, and AFCI in the protective device.
For these and other like reasons, there has been an increase in the volume of the protective device's housing in order to meet new requirements. On the other hand, because of all of the trends discussed above, including the decrease in size of wall studs and wall board, the size of the outlet box has decreased as well. Unfortunately, the available volume inside the outlet box for the electrical wiring is reduced accordingly. This reduction in available volume results in greater susceptibility to one or more of the installation problems described above.
What is needed is a smaller protective device that provides all of the protective safety features currently provided by larger devices. It is further desirable to provide a protective device having a width behind the strap of less than one inch.